The Stovebolt Site Forums The Shop Area The Electrical Bay Ignition coil and resistor question

"Full" battery voltage may be down around 10 volts while the starter is engaged. When you are starting the engine, full battery voltage is switched to coil+ to provide a strong spark.

Once the engine starts and you release the key the voltage through the resistor is applied to coil+. The resistor reduces the DC battery voltage to around 8 volts at the coil+ when the points are closed. This reduces the current (amps) going through the coil primary winding extending coil and points life.

When the points first open the magnetic field in the coil collapses providing lots of volts from the secondary winding in the coil, out through the big wire to the distributor and on to one of the spark plugs. Meanwhile, back at the points there is a very short electrical ringing (see previous post's attachment) that arcs across the barely open points. That arcing over many miles is what causes "burning" of the points. When the points "burn" enough over time, the resistance through the points increase which reduces the amount of voltage drop and current through the coil primary, which reduces the amount of spark at the spark plug. The condenser's job is to short as much of that high voltage ringing to ground while still maintaining the 8 volt or so of DC voltage at the connection between coil- and the open moveable point.